The present invention relates to suspensions and, more particularly to suspensions for vehicles, such as sedans. More specifically, it concerns means used to allow motion between two points (a suspended point, connected for example to the body or chassis of a car, and a non-suspended point such as a point which is part of a wheel carrier) while striving to reduce any variations in the force developed by the suspension seen from the standpoint of the suspended point.
It is known that the suspension of a vehicle includes design features by means of which the displacement of a wheel or wheels of an axle with respect to the chassis (or with respect to the body of the vehicle) is guided, and it also includes means which provide the suspension with a certain degree of flexibility and damping. For purposes of convenience, the latter means can be referred to as the spring function and the damping function. More generally, these means act on the movements existing between the wheel and the chassis. It should be noted at the outset that throughout this document, in order to simplify our presentation, only the type of vehicle suspension referred to as the vertical type is considered. Of course, this particular context does not limit the invention, even though it constitutes a preferred area thereof.
A spring is calculated so as to support the load of the vehicle while maintaining a sufficient clearance between the vehicle and the ground. Due to the damping function, wheels equipped with tires tend to remain in continuous contact with the road when the suspension is subjected to stress by irregularities in the road. A shock absorber prevents any oscillations occurring following a disturbance from being maintained, such as oscillations of the wheel itself, which the tire cannot attenuate by itself with sufficient speed.
It is known that the design of a suspension is characterized by the search for a compromise between objectives which have the reputation of being contradictory, at least to a large extent: comfort and road holding. Numerous efforts have been made in order to find increasingly favorable compromises. In prior art, there are suspensions referred to as active or semi-active whose control has now been made possible by progress in electronics. In such suspensions, commands are transmitted by electric or electrohydraulic means in order to modify the flexibility and/or damping characteristics of the suspension on a real-time basis. This leads to the addition of an electronic regulation stage, with sensors and means for calculation, to conventional mechanical, pneumatic, or hydraulic devices, as well as electronically-controlled valves or servovalves, in order to transmit the commands for changing the characteristics to the mechanical devices responsible for the functioning of the suspension as such. For example, a valve in a shock absorber is opened or closed in order to modify the dissipation of energy it causes or a valve is opened or closed to put a supplementary hydropneumatic accumulator in or out of circuit in order to modify the overall stiffness of the spring.
Unfortunately, this makes the basic components of the suspension, i.e., the springs and the shock absorbers, much more complex. In addition, although it is desirable that the suspension be capable of instantaneously adapting its characteristics to modifications in road conditions encountered by the vehicle, the electronically-controlled valves or servovalves are responsible for a delay in changing the actual state of the characteristics of the suspension due to their response time. The use of electronically-controlled valves or servovalves thus imposes a technological limit which makes it impossible to achieve the desired reaction speed. It appears difficult to expect response times below one hundredth of a second.
This is the reason for which designers of active or semi-active suspensions develop regulation strategies which are capable of anticipation; the control of the suspension is based on measurements of the speed of the vehicle, the angle and/or rotation speed of the steering wheel, the braking pressure and/or rate of increase in braking pressure, and the degree and/or speed of depression of the accelerator pedal. These parameters make it possible to anticipate to the extent possible the probable movements of the body which would be caused by the actions of the driver by means of experimental observation of vehicle behavior.
Although this anticipation may be interesting to a certain degree, it remains insufficient for rapidly taking into account disturbances which are unrelated to the operation of the vehicle, such as disturbances resulting from the bumpy nature of the ground on which one is driving. This is why attempts have been made to more directly control the inherent characteristics of suspension devices using electricity. Unfortunately, there are few technological possibilities of modifying the force introduced by a shock absorber in this manner. Furthermore, there is no known simple method of directly adjusting the stiffness of a spring by means of an electric current, voltage, or electro-magnetic field. Nonetheless, it remains true that a suspension fundamentally comprises a spring. It is therefore also desirable to be able to act on the spring function in order to be able to improve comfort while maintaining driving safety, i.e., handling performance (and therefore permanent contact between the wheel and the ground, even under extreme conditions).
In addition, attempts have already been made to adopt means for acting on a suspension which are better suited for electrical control. US Pat. No. 5,060,959 presents an electrical actuator which acts in parallel to a pneumatic spring to control the movements of the wheel of the vehicle. Nevertheless, this control makes use of numerous sensors of forces and position, making its practical utilization somewhat problematic. In addition, it turns out that the electrical actuator always. behaves as a conventional shock absorber in the rebound phases of the suspension (a rebound is a movement of the wheel carrier away from the chassis) as it opposes the movement of the suspension. This approach is not satisfactory for improving vehicle comfort.
The prior art has therefore been faced with the choice of control parameters and the correct use of a multitude of signals felt to be necessary for correctly determining the attitude of a vehicle, and finally, although progress in electrical control has given rise to hopes for the past several years of progress in the area of vehicle suspension, there are still problems with mastering the characteristics of an active suspension.